Implantable neuromodulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
These implantable neuromodulation systems typically include one or more electrodes carrying stimulation leads, which are implanted at the desired stimulation site, and an implantable neuromodulation device (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the neuromodulation lead(s) or indirectly to the neuromodulation lead(s) via a lead extension. The neuromodulation system may further comprise a handheld external control device (e.g., a remote control (RC)) to remotely instruct the neuromodulator to generate electrical stimulation pulses in accordance with selected modulation parameters.
Implantable neuromodulation devices are active devices requiring energy for operation, and thus, the neuromodulation system oftentimes includes an external charger to recharge a neuromodulation device, so that a surgical procedure to replace a power depleted neuromodulation device can be avoided. To wirelessly convey energy between the external charger and the implanted neuromodulation device, the charger typically includes an alternating current (AC) charging coil that supplies energy to a similar charging coil located in or on the neuromodulation device. The energy received by the charging coil located on the neuromodulation device can then be stored in a rechargeable battery within the neuromodulation device, which can then be used to power the electronic componentry on-demand. Depending on the settings, the neuromodulation device may need to be recharged every 1-30 days.
Electrical stimulation energy may be delivered from the neuromodulation device to the electrodes in the form of an electrical pulsed waveform. Thus, stimulation energy may be controllably delivered to the electrodes to stimulate neural tissue. The configuration of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode configuration, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode configuration represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, pulse width, and rate (or frequency) of the electrical pulses provided through the electrode array. Each electrode configuration, along with the electrical pulse parameters, can be referred to as a “modulation parameter set.”
The lead or leads are typically placed in a location, such that the electrical stimulation will cause paresthesia. The current understanding is that paresthesia induced by the stimulation and perceived by the patient should be located in approximately the same place in the patient's body as the pain that is the target of treatment. If a lead is not correctly positioned, it is possible that the patient will receive little or no benefit from an implanted SCS system. Thus, correct lead placement can mean the difference between effective and ineffective pain therapy. When electrical leads are implanted within the patient, the computerized programming system, in the context of an operating room (OR) mapping procedure, may be used to instruct the neuromodulation device to apply electrical stimulation to test placement of the leads and/or electrodes, thereby assuring that the leads and/or electrodes are implanted in effective locations within the patient.
Although alternative or artifactual sensations are usually appreciated by patients, patients sometimes report these sensations to be uncomfortable, and therefore, they can be considered an adverse side-effect to neuromodulation therapy in some cases. It has been shown that the delivery of sub-threshold electrical energy (e.g., high-rate pulsed electrical energy and/or low pulse width electrical energy) can be effective in providing neuromodulation therapy for chronic pain without causing paresthesia.
Once the leads are correctly positioned, a fitting procedure, which may be referred to as a navigation session, may be performed using the computerized programming system to program the external control device, and if applicable the neuromodulation device, with a set of modulation parameters that best addresses the painful site. Thus, the navigation session may be used to pinpoint the volume of activation (VOA) or areas correlating to the pain. Such programming ability is particularly advantageous for targeting the tissue during implantation, or after implantation should the leads gradually or unexpectedly move that would otherwise relocate the stimulation energy away from the target site. By reprogramming the neuromodulation device (typically by independently varying the stimulation energy on the electrodes), the volume of activation (VOA) can often be moved back to the effective pain site without having to re-operate on the patient in order to reposition the lead and its electrode array. When adjusting the volume of activation (VOA) relative to the tissue, it is desirable to make small changes in the proportions of current, so that changes in the spatial recruitment of nerve fibers will be perceived by the patient as being smooth and continuous and to have incremental targeting capability.
An external control device can be used to instruct the neuromodulation device to generate electrical stimulation pulses in accordance with the selected modulation parameters. Typically, the modulation parameters programmed into the neuromodulation device can be adjusted by manipulating controls on the external control device to modify the electrical stimulation provided by the neuromodulation device system to the patient. Thus, in accordance with the modulation parameters programmed by the external control device, electrical pulses can be delivered from the neuromodulation device to the stimulation electrode(s) to stimulate or activate a volume of tissue in accordance with a set of modulation parameters and provide the desired efficacious therapy to the patient. The best modulation parameter set will typically be one that delivers stimulation energy to the volume of tissue that must be stimulated in order to provide the therapeutic benefit (e.g., treatment of pain), while minimizing the volume of non-target tissue that is stimulated.
The clinician generally programs the neuromodulation device through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC). The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the neuromodulation device to allow the optimum modulation parameters to be determined based on patient feedback or other means and to subsequently program the neuromodulation device with the optimum modulation parameter set or sets. The computerized programming system may be operated by a clinician attending the patient in several scenarios.
One known computerized programming system for SCS is called the Bionic Navigator®, available from Boston Scientific Neuromodulation Corporation. The Bionic Navigator® is a software package that operates on a suitable PC and allows clinicians to program modulation parameters into an external handheld programmer (referred to as a remote control). Each set of modulation parameters, including fractionalized current distribution to the electrodes (as percentage cathodic current, percentage anodic current, or off), may be stored in both the Bionic Navigator® and the remote control and combined into a stimulation program that can then be used to stimulate multiple regions within the patient.
A typical transverse section of the spinal cord will include a central “butterfly” shaped central area of gray matter (neuronal cell bodies) substantially surrounded by an ellipse-shaped outer area of white matter (myelinated axons). The dorsal horns are the dorsal portions of the “butterfly” shaped central area of gray matter, which includes neuronal cell terminals, neuronal cell bodies, dendrites, and axons. Conventional SCS programming has as its therapeutic goal maximal stimulation (i.e., recruitment) of dorsal column fibers that run in the white matter along the longitudinal axis of the spinal cord and minimal stimulation of other fibers that run perpendicular to the longitudinal axis of the spinal cord (dorsal root fibers, predominantly). The white matter of the dorsal column includes mostly large myelinated axons that form afferent fibers.
While fibers in the dorsal column run in an axial direction, fibers in the dorsal horn can be oriented in many directions, including perpendicular to the longitudinal axis of the spinal cord. Dorsal horn fibers are also a different distance from the typically placed epidural SCS leads, when compared to dorsal column fibers.
Further, dorsal horn fibers and dorsal column fibers have different responses to electrical stimulation. The strength of stimulation (i.e., depolarizing or hyperpolarizing) of the dorsal column fibers and neurons is described by the so-called “activating function” ∂ 2V/∂ x2 which is proportional to the second-order spatial derivative of the voltage along the longitudinal axis of the spine. This is partially because the large myelinated axons in dorsal column are primarily aligned longitudinally along the spine. On the other hand, the likelihood of generating action potentials in dorsal horn fibers and neurons is described by the “activating function” ∂ V/∂ x (otherwise known as the electric field). The dorsal horn “activating function” is proportional not to the second-order derivative, but to the first-order derivative of the voltage along the fiber axis. Accordingly, distance from the electrical field locus affects the dorsal horn “activating function” less than it affects the dorsal column “activating function.”
Current implantable neuromodulation systems typically include electrodes implanted adjacent to the dorsal column of the spinal cord of the patient. Current implantable neuromodulation systems are also typically programmed to deliver stimulation energy to the spinal without differentiating between the dorsal column and the dorsal horn of the spinal cord of the patient. While generally stimulation of neuronal elements (e.g., neurons, dendrites, axons, cell bodies, and neuronal cell terminals) in the patient's spinal cord provides therapy for pain, such stimulation sometimes causes alternative or artifactual sensations (e.g., paresthesia), which are sometime unwelcomed by the patient. Such stimulation also requires (1) selective lead placement, as described above, (2) optimal stimulating electrode selection, and (3) optimization of electrode configuration (e.g., polarity and anode-cathode separation). Accordingly, these exists a need for implantable neuromodulation systems and modulation parameter sets for same that provide therapy for pain while minimizing alternative or artifactual sensations and the sensitivity of the system to modulation parameters. There also exists a need for an implantable neuromodulation systems and modulation parameter sets for same that preferentially stimulate dorsal horn neuronal elements over dorsal column neuronal elements.
Current implantable neuromodulation methods also include an electrical field localization step, in which the longitudinal location of the electrical field locus is identified through trial and error and with patient feedback. While electrical field localization provides effective neuromodulation, the process is time-consuming and requires patient participation. Further, when an electrical modulation lead shifts, repeating electrical field localization may be required. Accordingly, there exists a need for implantable neuromodulation systems and parameter sets for effective neuromodulation while minimizing the need for electrical field localization.